A single- or multicore combination of cores (single lines) sheathed by insulation materials that is used to transmit energy or information is generally termed a cable. Different plastics are usually used as insulation materials, which enclose the cores used as conductors and insulate them from one another. Electrical conductors mostly consist of copper, less frequently also of aluminium or suitable metal alloys. Viewed three-dimensionally, the cable follows a mostly cylindrical or similar geometry and can in its overall construction also contain further sheath layers of insulating material or metallic foils, or braids for the purpose of electromagnetic shielding or as mechanical protection.
High currents in the order of several amperes and above necessitate a suitably large conductor cross section. In the transmission of alternating signals, such as e.g. alternating current, the current in the conductor cross section is displaced to the conductor surface by internal magnetic fields as the frequency rises. This effect is known as the skin effect. For example, at a frequency of 10 MHz the current density 20 μm below the surface is only the 1/eth part (37%) of the current density on the outermost surface. This means that only a small portion of the overall cross section of the cable carries the main part of the current.
It is known to use cables with litz wires in many electrical engineering fields. A litz wire in electrical engineering is an electrical conductor consisting of thin individual wires. Litz wires are often easy to bend. In electrical cables, copper is often used as a conductor for this. The individual wires of the litz wires (e.g. several hundred individual wires) are mostly enclosed by a common insulating sheath. A conductor formed in this way is normally termed a stranded lead or stranded conductor. If several such lines are combined in a cable, they are often described as cores of the cable.
Compared with solid wire, high-frequency litz wires, the individual conductor surfaces of which are insulated from the other wires of the litz wire, have a higher quality in the high-frequency range. This is based on the enlargement of the cross section effectively involved in the current flow, which cross section is limited in solid wire by the skin effect already cited and also by the so-called proximity effect. The proximity effect is based on the current displacement between two closely adjacent conductors. In normal litz wires, i.e. not high-frequency litz wires with insulated individual wires, the conductors are in contact with one another and the skin effect acts as in solid conductors. In addition, due to the longitudinal propagation of the current and the litz wires turning away underneath, an additional contact resistance is to be found. For this reason normal litz wires tend to be poorer at high frequency (HF) than solid conductors.
To reduce the skin effect and/or the proximity effect, insulation is provided as mentioned between the individual wires in high-frequency litz wires (normally shortened to HF litz wires). Lacquer is often used for the insulation, i.e. the individual wires of a litz wire are insulated from one another by a lacquer coating. This insulation is also provided accordingly if the litz wires carry the same potential. By reducing the skin effect and/or the proximity effect, a larger portion of the overall cross section of the cable participates in transporting the current. However, this approach is elaborate in the manufacture and processing of the cable. In addition, a cable with a complex structure is created.
The requirement exists to provide a simply constructed cable with good properties, such as e.g. good overall efficiency and/or minimum energy consumption and/or minimum self-heating and/or best electromagnetic compatibility.